In the past when we had this happen in my previous organization, it was really handled no differently then any other change request that was coming into the system, a change is a change. What we did was define who would be the Owner of the change in our system on behalf of the vendor(since the vendor didn't have access to our tracking tool). So if you have account managers that may be the best approach. Then if there were any questions regarding the technical approach or timing etc we would involve both parties, internal & external. If the risk/impact was significant or higher, representatives from the vendor would attend our cab's. This is one approach of I'm sure many.

I suppose the idea is to treat it as a different way of entering the start of the process and to consider the 3rd party that will carry out the changeas just another resource.

Yes, definitely. They (the vendor) now represent a "work dependency". When you're planning the work done for a Change/Release, you are performing Project Management functions by coordinating work and all the work dependencies necessary to get that work done. Engaging the vendor will now be a standard part of the work dependencies, whenever you want to make a Change that they must be involved in.